Electrophotographic-type processes involve devices one of whose components includes a layer of photoconductive insulating material fixed to a conductive backing to form a film structure termed a "photoconductor". Initially, the photoconductor is uniformly electrostatically charged over its entire surface, following which it is exposed to a light pattern corresponding to an image to be reproduced. The charge on those surface areas impacted by the light of the image is thereby relatively dissipated, leaving only areas not so impacted in a charged condition. The charge remaining on the surface, therefore, conforms to the configuration of the light pattern reflected from the image to be reproduced.
This latent electrostatic image on the photoconductor can be subsequently developed by exposing it to finely divided, electrostatically attractable, particulate material. The material is drawn to such surface areas in amounts proportional to the magnitude of the charge in the electrostatically affected areas, thereby forming a temporary image of the material being copied.
The particulate material used to create the temporary image on the photoconductor, referred to in the industry as "toner", typically consists of a pigmented, thermoplastic resinous material which can subsequently be transferred to a supporting substrate on which the image of the document being copied is to be permanently fixed. Such a transfer can be accomplished, for example, with a corona discharge device that produces a charge on the substrate, opposite in nature to the charge of the toner forming the temporary image. Subsequent transfer of the temporary toner image to the substrate by electrostatic attraction occurs when the substrate and the photoconductor with the temporary image thereon are brought into proximity with each other. The transferred image can thereafter be fixed to the substrate by fusing the toner thereto, using any of the several known methods.
The photoconductor employed by the process commonly takes the form of an endless "belt" propelled by drive means which moves the belt through the various stages of the process. In the course of such progression, it is of importance that the rate of travel of the belt, and therefore, the time within which the various processing steps are performed be carefully controlled, otherwise improperly toned, and/or distorted images can result.
In order to make such control possible in electrophotographic machines of the type described, resort is frequently had to the use of an optical encoder device. The encoder comprises a disc provided with radial slots therein, having a light source positioned on one side thereof, and a light detector on the other. The perforated disc rotates on a shaft on which a sprocket is also mounted, the teeth of the sprocket engaging and being driven by perforations provided in the photoconductor. The frequency of detected light impulses as the perforated disc rotates, therefore, provides an indication of the characteristics of the belt travel. This information in the form of electrical impulses is transferred to the machine's logic control center, which then adjusts the operating parameters of the machine accordingly, in order to meet desired values.
While machine control is effectively accomplished with the encoder system described, the system is not, however, without problems. In this regard, maintaining contact of the sprocket teeth with a particular edge of the perforations in the photoconductor, for example the trailing edge, is a difficult task due to fluctuations in the velocity, acceleration, etc. of the photoconductor film. These variations cause the position of the teeth of the sprocket, relative to the perforations, to be quite variable. The result is that at a particular point in time, a given tooth is in contact with the trailing edge of a perforation, while at another, it is forced against the perforations leading edge. This constant movement back and forth within the perforations produces impacts which over time result in destructive damage to the film perforations, as well as in inaccuracies in the detection process, creating consequential problems relating to machine control, and causing the production of defective copies.
While these difficulties could to a certain degree be avoided through the provision of a frictional drag device operating on the sprocket-optical encoder assembly which would tend to maintain the trailing edge of the sprocket teeth against the trailing edge of the perforations, unfortunately, frictional devices tend to undergo gradual wear, causing increasingly erratic braking behavior. Furthermore, such brakes often exhibit variable coefficients of friction in changing environments, and are subject to substantial variability in their braking characteristics, particularly at the low force values encountered in the case of the optical encoder devices contemplated by the invention.